1. Field of the Invention
The present invention generally relates to color television receivers, and particularly relates to a magnetic shielding system in a color television receiver including a combination of magnetic shielding and magnetic cancelling as measures to counter external magnetism, particularly applicable to an extra large color cathode ray tube in a high-definition television.
2. Description of the Prior Art
In order to counteract undesirable external magnetic disturbances such as geo-magnetism, or the like, a color cathode ray tube 1 (hereinafter abbreviated to CRT), as generally used today, often includes an internal magnetic shield 1b (hereinafter abbreviated to "IMS") connected to a shadow mask 1a as shown in FIG. 1A. Generally, the IMS 1b is composed of iron plates and arranged to cooperate with a magnetic erasing coil (not-shown) provided outside the color CRT 1 to provide a magnetic shielding effect so as to make an external magnetic field hardly influence the electron beams in the color CRT 1. As a measure to counteract external magnetism for a color CRT, however, there had been used, as shown in FIG. 1B, an external magnetic shield 6 (hereinafter abbreviated to "EMS") of the type provided outside the color CRT 1. The EMS 6 is fixed to the CRT 1 by utilizing the holes of fittings 7 attached to the color CRT 1. The EMS system had been used prior to the widespread use of the so-called IMS system employing the above-mentioned IMS 1b. In each of FIGS. 1A and 1B is shown a deflecting yoke 5. The EMS 6 covers the major part of a funnel 1F of the color CRT 1 but does not cover the major part of a forward panel 1P, so that the magnetic shielding effect is insufficient. In addition to its insufficient magnetic shielding effect, the EMS system is not only bulky in the color television receiver but is high in cost and troublesome to assemble. As a result, the EMS system has gradually fallen out of use, and, at present, the IMS system shown in FIG. 1A is widely used in it place.
Recently, a desire to enlarge color CRTs has become intense, particularly for high-definition television receivers, e.g., CRTs of about 40 inches have been developed. As color CRTs are made large in size, the propagation distance of electronic beams between an electron gun and a screen in the respective CRT becomes extremely large so as to make it very difficult to counteract external magnetism. FIG. 2 is a diagram for explaining the relationship between the kind of magnetic shielding system and the travelling distance of electron beams in a 37 inch color CRT having a deflection of 110 degrees. In FIG. 2, the distance of shift in incidence of electron beam in the horizontal direction on a fluorescent screen at a corner of a picture screen is plotted when an external magnetic field is changed by 0.8 gauss. The reason why the quantity of horizontal shift of electron beams is shown in FIG. 2 is that the fluorescent screen of the 37-inch 110 deflection color CRT is assumed to have a stripe structure. However, not only the quantity of horizontal shift of electron beams, but the distance of vertical shift of the beam must be considered in the case where the color CRT has a dot structure. Nonetheless, the discussion below will be limited to the case of the color CRT having a stripe structure. In FIG. 2, the symbol E/W designates a magnetic field in the direction of east and west, N/S a magnetic field in the direction of south and north, and BV a magnetic field in the vertical direction, as defined by the enclosed perspective view of the CRT 1. In FIG. 2, it is apparent that the non-IMS and non-EMS system having neither IMS nor EMS cannot be used because the shift distance of an electron beam is extremely large except in the E/W direction.
Similarly, in FIG. 2, in the conventional IMS system of the color CRT 1 in the color television receiver, the shift distance of the electron beams has been considerably suppressed but does not fall below 150 micrometers, which is the threshold in practical use. This threshold is indicated by a wavy line in FIG. 2. The threshold of shift distance is different depending on the kind of color CRT 1. For example, in the 37-inch 110 deflection color CRT, the above-mentioned threshold (150 micrometers) of shift distance is a value which may be a standard for judging whether the color CRT can be used practically or not. Accordingly, the problem still exists in the EMS system shown in FIG. 1B such that the EMS system can not be practically used because the shift distance of the electronic beams is larger even than that in the IMS system shown in FIG. 1B.